\usepackage{ifthen}

\newcommand{\re}{\ensuremath{\mathbf{R}}}
\newcommand{\nat}{\mathbf{N}}
\newcommand{\zahl}{\mathbf{Z}}
\newcommand{\rat}{\mathbf{Q}}
\newcommand{\thingy}[2][m]
{\ifthenelse{\equal{#1}{m}}{\medskip}{\smallskip}\noindent\textbf{#2}\quad}
\newcommand{\eps}{\epsilon}
\newcommand{\eff}{\mathcal{F}}
\newcommand{\implies}{\Rightarrow}
\newcommand{\T}[1]{T(\mathsf{#1})}

\newcommand{\splitfunc}[4]{
\left\{ \begin{array} {r@{\quad:\quad}l}
#1 & #2 \\ #3 & #4 \end{array} \right.}